add missing newline

spec/spec_helper.rb

@@ -20,4 +20,4 @@ if ENV['TRAVIS']
   exit 0 if ENV['TRAVIS_PULL_REQUEST'] != 'false' && ENV['TRAVIS_BRANCH'] == 'master'
 end
 
-DEFAULT_STACK = 'heroku-16'
\ No newline at end of file
+DEFAULT_STACK = 'heroku-16'